The overall purpose of the proposed research is to determine why organisms are unable to remember events that occur during their infancy. Infantile amnesia has most often been attributed either to environmental processes that also affect adult memory or to processes arising from neurological changes during development. Of the specific hypotheses that have been proposed, most have been concerned with effects that occur after the event has been initially learned. However, very recent data from our laboratory suggest that long-term memory deficits for infant but not adult experiences may be determined during the learning process itself, which means that the phenomenon may ultimately be due to neurological immaturity. Because it is extremely difficult to study infantile forgetting with human subjects, most of the research conducted to date have involved infrahuman subjects, primarily the rat. Our basic procedure is to repeatedly expose preweanling rat pups to an auditory stimulus which predicts the occurrence of an aversive event and then to test the effect of this stimulus upon some easily-measured on-going behavior immediately after the conditioning experience or after a delay. The advantage of this procedure is that it is simple and robust, and presumably unaffected by environmental contributions to forgetting. We have found that this experience is not forgotten when experienced at an age which other investigators have found to be highly susceptible to later forgetting, whereas it is rapidly forgotten when experienced at a somewhat younger age. However, with certain simple changes introduced before forgetting has occurred, we can produce later memory deficits in older pups and alleviate those deficits in younger pups. Thus, our specific objectives are to clarify, extend, and relate these findings to a general theory of memory development.